Your plastic self

Who – or what – do you think you are? You probably think that your memories and personality are an important part of what you call your “self,” and you’d be right. But the core of your sense of self is something that you probably take completely for granted – your body.

Philosophers have always known about awareness of the body of the body is critical to the sense of self. In his 1739 book, A Treatise on Human Nature, for example, David Hume wrote, “When I enter into myself, I always stumble on some particular perception or other, or heat or cold, light or shade, love or hatred, pain or pleasure. I never can catch myself at any time without a perception.”

Hume clearly understood the importance of the body for the sense of self, as did various other philosophers who came later, and neuroscientists are now beginning to catch up with them. In the past 10 years, they have made big advances towards understanding the neurological basis of bodily awareness, revealing the mechanisms by which the brain makes – and can break – our sense of self-identity. Leaders in this area of research described their work today in a symposium held at the BNA Festival of Neuroscience.

Henrik Ehrsson of the Karolinska Institute in Stockholm described his work on body ownership, one of the two key components of bodily awareness. How do you know that your body belongs to you? The question sounds trivial, but the brain mechanisms underlying the sense of ownership are in fact extremely complex, and very malleable.

The best known example of the malleability of the sense of ownership is the so-called Rubber Hand Illusion (RHI). As the video above shows, this illusion can be induced very easily, by simply creating a discrepancy between what you see and what you feel. When the participant’s hidden hand is touched in exactly the same way and at the same time as the rubber hand, it creates the illusion that the sensations are originating from the fake hand instead of the real one. The brain incorporates the rubber hand into its model of the brain, while neglecting the real hand. In other words, the participant takes ownership of the rubber hand, while losing ownership of their own.

Ehrsson extended this to develop the Body Swap Illusion, which involves the same basic principles, but requires head-mounted displays with video cameras attached to them. If two people don this apparatus and stand opposite each other, so that each display shows real-time footage from the other’s camera, both see their own body from the third-person perspective, standing several feet ahead, in the position of the other person. If both of their bodies are then touched synchronously, as in the RHI, this induces in both of them the illusion that the sensations they are feeling are coming from the other person’s body. The Body Swap Illusion can also be induced with a life-like mannequin instead of a second participant, but not with a plank of wood. Again, this tells us that the brain can easily be fooled into thinking that an inanimate object is a part of the body, but that the object must to some extent resemble the body for it to do so.

Mel Slater of the University of Barcelona has shown that these illusions can also be induced in the virtual world. His work shows that people readily take ownership of a computer-generated arm, or even a whole body, that is projected onto a screen in front of them. Remarkably, Slater has also discovered that this process – referred to as embodiment – can change the way in which people behave and think. When one has their sense of ownership transferred to the virtual body of a child, for example, their behaviours and attitudes change accordingly, becoming more child-like during the illusion.

Far from being fancy party tricks, these illusions of bodily awareness offer profound insights into the neural basis of the sense of self. Together, they show that awareness of one’s body depends on the integration of different types of sensory information, particularly vision, touch, and proprioception, or muscle sense. This takes place in a specific region of the parietal lobe, which then uses the information to generate a mental picture of the body. This process is highly dynamic. It depends on a constant flow of sensory information from the body to the brain. As these illusions demonstrate, interrupting one of these sensory channels creates discrepancies in the brain, which can have bizarre consequences.

Research into the cognitive neuroscience of bodily awareness also lends itself to many clinical applications. An understanding of the neural mechanisms of body ownership will, for example, help researchers to develop prosthetic limbs that provide sensory feedback to the brain and become fully integrated into its model of the body, so that they actually feel like a part of the body instead of a cumbersome add-on. They also provide insight into conditions such as somatoparaphrenia, a disorder in which patients deny that one of their body parts belongs to them; anosagnosia for hemiplegia, in which patients deny that one side of their body has been paralysed by stroke; and Body Integrity Identity Disorder, in which patients have a burning desire to amputate a healthy limb.

Mo Costandi trained as a developmental neurobiologist and now works as a freelance science writer. He writes the Neurophilosophy blog, which is hosted by the Guardian, and his first book, 50 Human Brain Ideas You Really Need to Know, will be published in July.

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